Fluorescence Quenching and Polarization Studies of Naphthalene and 1-Naphthol Interaction with Humic Acid

Although it is known that the environmental behavior and fate of synthetic organic compounds are altered on association with humic materials, the nature of this interaction has been a source of controversy. Fluorescence quenching and polarization techniques were used to study the interaction between water-soluble humic acid and naphthalene and 1-naphthol. Stern-Volmer plots constructed using intensity values for these fluorophores dissolved in humic acid solutions (0–25 mg L⁻¹) were linear. The ultraviolet/visible (UV/VIS) absorption spectra of each fluorophore in the presence and absence of the humic acid quencher were identical. This, in addition to the observed increase in quenching with temperature, indicates a dynamic (collisional) mode of quenching. Fluorescent lifetimes of the probes decreased in the presence of humic acid, providing further evidence of dynamic quenching. However, the slopes of Stern-Volmer plots constructed with lifetime values were considerably smaller than plots obtained with intensity values. Calculated bimolecular quenching constants of 5.2 × 10¹⁰ M⁻¹ s⁻¹ for naphthalene and 4.8 × 10¹¹ M⁻¹ s⁻¹ for 1-naphthol are above the maximum considered possible for a diffusion-controlled interaction. Fluorescence-polarization measurements indicate no rigid association between unquenched 1-naphthol and humic acid. The interaction of naphthalene and 1-naphthol with humic acid in aqueous solution occurs through a loose association in which humic acid surrounds the fluorophore in a cage-like manner. This pseudomicelle confines the probe without rigidly binding it, promoting frequent quenching collisions and a combination of dynamic and static quenching. Funding provided by Idaho Water Resources Research Inst. (Grant no. G1419-04), Western Region Pesticide Impact Assessment Program, Univ. of Idaho Research Council, and the Utah Agric. Exp. Stn.